One modem wireless telecommunications system that is presently under development is commonly referred to as a third generation (3G) wideband code division/multiple access (WCDMA) system. It is proposed in the 3G-WCDMA system to use closed loop, feedback mode transmitter diversity. FIG. 1A depicts a presently proposed downlink (base station (BS) to user equipment (UE)) transmitter structure 1 for supporting closed loop transmit diversity. In FIG. 1 DPCH refers to a dedicated control channel and CPICH refers to common pilot channel. The DPCH includes a (dedicated) pilot signal channel (DPCCH) and a data signal, which is channel coded and interleaved before being spread and scrambled. The spread complex valued signal is fed to two transmitter antenna branches (Ant1 and Ant2), and weighted with antenna-specific weight factors w1 and w2, respectively. The weight factors are complex valued signals. The weight factors (actually the corresponding phase adjustments) are determined by the UE 2, and are signaled to the BS 1 through an uplink DPCCH.
There are actually two feedback modes, which are uniquely identified by the mode specific antenna weight value set. The structure of the feedback signaling message (FSM) is shown in FIG. 1B, where it can be seen that the FSM has two parts. The first part of the FSM is FSMph, which transmits the phase setting, while the second part is FSMpo, which transmits the power setting. FIG. 1C is a table that summarizes the characteristics of the feedback mode, where NFBD is the number of feedback bits per slot, NW is the length of the feedback command in slots, update rate is the feedback command rate, Npo is the number of power bits, and Nph is the number of phase bits per signaling word.
The UE 2 uses the common pilot channel CPICH to separately estimate the channels seen from each antenna (Ant1 and Ant2). Once every slot the UE 2 chooses amongst the mode-specific transmit weight set an optimum weight which, when applied at the BS 1, maximizes the received power at the UE 2. The UE 2 then feeds back to the BS 1 the FSM, which informs the BS 1 of which power/phase settings should be used. If Npo is zero, then equal power is applied to both transmit antennas.
In the first feedback mode equal power is applied to Ant1 and Ant2, and the UE 2 must then only determine the phase adjustment between Ant1 and Ant2. This is done using channel estimates computed from the CPICH. The BS 2 averages the phases of two consecutive slots, implying that the possible transmit weights in the feedback mode 1 are exactly the QPSK constellation points.
It can thus be appreciated that in the closed loop transmit diversity mode of operation the user equipment 2 determines an optimal phase shift for the BS transmission antennas (Ant1 and Ant2) and transmits a (one bit) feedback (FB) command to the BS. The BS uses two successive one bit FB commands to determine transmission weights w1 and w2 for antennas Ant1 and Ant2, respectively, when transmitting the DPCCH to the UE 2. The weight for Ant1 (w1) is always unity, while the weight for Ant2, w2, has values of eiφ, where φε{π/4, 3π/4, −π/4, −3π/4}.
In the proposed embodiment of FIG. 1A the BS 1 uses orthogonal common pilot patterns for the CPICH channels of antennas 1 and 2. These channels are common to all UEs 2 in the cell area, and are transmitted without UE-specific transmission weights. However, a UE 2 will typically wish to utilize CPICH channels in channel estimation due to the higher transmission power resulting in a more reliable channel estimation. In order to properly combine the channel estimates corresponding to CPICH1 and CPICH2, the UE 2 must know the transmission weight w2 that was utilized by the BS 1. A more detailed description of the mode 1 closed loop transmit diversity can be found in the 3GPP specification TS25.214:“Physical layer procedures (FDD)”.
However, the feedback channel that is used to transmit the FSMs from the UE 2 to the BS 1, which are employed by the BS 1 to steer the phase shift of antenna 2, is not error free. As such, due to reception errors in the feedback channel the BS 1 may not always transmit the DPCCH using the optimal phase shift determined by the UE 2. Since the UE utilizes the common pilot channels in the channel estimation it must know the value of w2, i.e., the phase shift applied at ANT2. Without this knowledge the UE 2 will combine the CPICH-based channel estimates assuming a zero phase shift for antenna 2, resulting in an incorrect channel estimate and a degradation in UE performance.
An example algorithm for the determination of the transmit antenna weight is presented in the above mentioned 3GPP specification TS25.214:“Physical layer procedures (FDD)”. The proposed algorithm utilizes a priori probabilities for the transmission weights, i.e., it assumes that the phase shift corresponding to a feedback command that was sent is more probable than a transmission weight corresponding to an inverted FB command (the FB command changed by an error in the feedback channel). The algorithm determines whether it is more probable that a feedback error occurred than that a feedback error did not occur and, based on this decision, the algorithm selects the transmission weight w2 corresponding to the sequence of transmitted feedback commands, modified according to determined feedback error occurrences.
This proposed algorithm requires knowledge of the relative power levels of the dedicated and common pilot channels, as well as knowledge of the variance of the noise plus interference. However, a reliable estimation of noise variance is a computationally expensive task and is thus undesirable for execution by the UE 2. While some form of variance estimation is required, in a rapidly changing signal propagation environment the averaging times should be made short, and thus the reliability of the estimate is low. The same considerations and problems apply to the estimation of the relative power levels of the dedicated and the common pilot channels, especially when the dedicated channel power is varied by a fast power control algorithm.
It can be appreciated that problems exist in the mode 1 closed loop transmit diversity operation described above.